1. Field of the Invention
The present invention related to the field of short-arc discharge lamps. More specifically, the present invention relates to short-arc discharge lamps that are used for exposure of liquid crystals, color filters and print substrates.
2. Description of Related Art
Short-arc discharge lamps have been well known as light sources for exposure used in the process of manufacturing liquid crystals, the process of manufacturing color filters, and the process of manufacturing print substrates. These short-arc discharge lamps typically have mercury sealed into their light-emitting tubes so that in operation, the mercury emits ultraviolet radiation, as shown for example in the Japanese Patent No. 2-256150.
In order to properly expose the liquid crystal substrate or color filter, ultraviolet energy of about 200 mj is required. In other words, if the luminous flux density at the area of exposure is to be 10 mW/cm2, the exposure will take 20 seconds.
Consequently, ways to shorten the exposure time have been considered as a method to increase productivity in the exposure process. That is, a stronger lamp for emitting the ultraviolet radiation has been sought to increase the luminous flux density at the area of exposure.
In recent years, there has been a trend toward larger liquid crystal display devices. Thus, demands for higher productivity and lower costs have brought about the use of such means as multifaceting using larger substrates. That is, as the substrates have become larger, the area for exposure has correspondingly increased, thereby necessitating use of stronger lamps to emit the ultraviolet radiation.
For the exposure of liquid crystals and print substrates, ultraviolet radiation known as g-rays, h-rays and i-rays have been used. The intensity of the ultraviolet radiation emitted by the lamps has been increased by sealing large amounts of mercury into the light-emitting tubes.
At present, lamps in the 8 kW class with about 15 mg/cc of mercury sealed into the light-emitting tubes are used as light sources for exposure.
For future productivity increases or enlargement of the substrate size, it will be necessary to have at least 15 mg/cc of mercury sealed into the light-emitting tube in order to increase the intensity of the ultraviolet light emitted.
In such lamps with at least 15 mg/cc of mercury in them, higher power will have to be input to the lamp to completely vaporize all the mercury in the light-emitting tubes. For that reason, it will be necessary to increase the anode temperature, and the current flow will generally be at least 50 A.
On the other hand, the ultraviolet radiation emitted from short-arc discharge lamps of this sort is focused on the area of exposure by mirrors and lenses within the exposure equipment. Thus, it is desirable that the arc spot be quite small in order to achieve good convergence of the ultraviolet radiation. As a result, the electrode separation between the cathode and anode has been shortened as a structural method to reduce the arc spot in discharge lamps.
Nevertheless, when the input power is increased in order to vaporize the large amount of mercury sealed into the light-emitting tube, there is inevitably a large current flow to the cathode which correspondingly causes the temperature of the cathode to rise.
Moreover, when the electrode separation is shortened to make the arc spot smaller, the space in which the energy is concentrated is also made smaller which correspondingly further increases the temperature of the cathode all the more.
Therefore, there has been a problem in that as the cathode temperature rises, the ultraviolet radiation emitted by the lamp begins to scatter so that the exposure light emitted lacks uniformity. Moreover, as the temperature of the cathode rises, the tip of the cathode vaporizes, and the tungsten of which the cathode is made, adheres to the inner wall of the light-emitting tube and blackens it. As a result of all of these factors, the amount of ultraviolet radiation passing through the light-emitting tube is quickly reduced and the intensity of ultraviolet radiation emitted by the short-arc discharge lamp drops. Ultimately, the short-arc discharge lamp must be replaced very frequently and provides a very short service life.
The short-arc discharge lamp in accordance with the present invention avoids the disadvantages of the discharge lamps described above. The present invention provides a short-arc discharge lamp with long service life and minimal scattering of the ultraviolet radiation emitted by the lamp. This is attained in accordance with the present invention, by suppressing the temperature rise in the cathode.
In order to resolve the problems described above, the short-arc discharge lamp in accordance with one embodiment of the present invention includes an anode and a cathode positioned facing each other within a light-emitting tube. The light-emitting tube is sealed an inert gas and at least 15 mg/cc of mercury. When the short-arc discharge lamp in accordance with the present invention is operated with electricity having a current value of at least 50 A, the ratio of Y/X is at least 500 (W/mm), the X being defined as the electrode separation between the anode and cathode in millimeters, and Y being defined as the input power in Watts. The outer surface of the cathode has a heat-release layer made of tungsten except at the tip.
In an alternative embodiment, the cathode may be provided with grooves on an outer surface, except at the tip, and a heat-release layer of tungsten over the grooves.